1. Field of the Invention
This invention relates to a device comprising two sensitive elements, one of which is a reference element, for determining the presence of gaseous hydrocarbons in a gas mixture. The construction of easy to read portable instruments able to detect methane or other gaseous hydrocarbons requires the development of highly sensitive low-cost sensors.
2. Description of the Related Art
Since it was discovered, about 30 years ago, that the adsorption of a gas on the surface of a semiconductor produces a variation in the electrical resistance of the material and that this effect disappears as soon as the causes which generate it are eliminated, it has been sought to utilize these properties in the construction of simple low-cost gas sensors. Semiconductor sensors, generally of tin oxide, were developed initially to provide an alarm signal for natural gas, carbon monoxide, hydrogen and alcohol vapours. Sensors of this type are described in Chemical Sensor Technology ed. Tetsuro Seijama Elsevier 1988. Perfecting this type of sensor has been rather slow and difficult. The reason for this is that a semiconductor sensor is intrinsically unselective, in that it is sensitive not to a single species but to all species which have the same surface impact on it. Other problems are connected with stability and reproducibility.
A common drawback of this type of sensor is that before being put into operation it requires a long period of conditioning at the operating temperature.
Selectivity, stability and reproducibility are the main problems which have to be solved for the large-scale introduction of these low-cost devices.
Stability and reproducibility can be improved mainly by improving the method of construction of the sensitive element, whereas improving selectivity involves not only seeking new materials or catalysts sensitive to a particular type of gas, but also constructing arrays as described in U.S. Pat. Nos. 4,457,161 and 4,542,640. The array is an assembly of sensors each of which reacts differently on interaction with a gas mixture, to thus provide different responses. By processing these responses a given gaseous chemical species present in the environment in which the sensor is immersed can be distinguished. However this response analysis is made difficult by the low stability and poor reproducibility of each individual sensor.
It has now been found that these drawbacks can be overcome by a gas alarm sensor device consisting of two sensitive elements in the form of thin tin oxide films applied by a silk-screen process, one of which being a reference element, and which are heated to different temperatures.